For the continuous production of PET, terephthalic acid is used as raw material. The essential process steps include the preparation of the raw materials, the esterification, the prepolycondensation and the polycondensation. In detail, terephthalic acid is mixed with ethylene glycol and a catalyst to obtain a paste and is then supplied to the esterification. Esterification is effected at atmospheric pressure by removing water. Prepolycondensation is effected under a vacuum. Polycondensation is performed at an elevated temperature and under an increased vacuum. The vapors containing water, acetaldehyde, 2-methyl-1,3-dioxolane, 1,4-dioxane and—in dependence on the catalyst used—acetic acid, which are formed in this process, are rectified, the ethylene glycol obtained as bottom product is recirculated to the esterification stage, and the lower-boiling components are discharged as top product and condensed. Due to the lower-boiling 2-methyl-1,3-dioxolane, which is formed in the production of PET from acetaldehyde and ethylene glycol by removing water, relatively large amounts of ethylene glycol are discharged indirectly in the top product. In the presence of an excess of water, the 2-methyl-1,3-dioxolane is again decomposed in dependence on the temperature, the pH-value and the reaction time to obtain the starting components acetaldehyde and ethylene glycol.
In the production of PET, an aliquot of the vapor condensate is usually recirculated to the rectification, and the excess aliquot is stripped once by means of air, the major part of the lower-boiling components being converted to the gas phase. The vapor condensate prepurified in this way is discharged for further treatment.
The exhaust air streams obtained during the production of PET are supplied to the air blower for stripping the excess vapor condensate. The exhaust air of the stripping process, which is laden with lower-boiling components, is supplied to a heat transfer furnace for thermal combustion.
According to a lecture of John Brown Deutsche Engineering, PET Resin Technology and Products, South Asia, Polyester & PET 96, Nov. 20 and 21, 1996, Mumbai, India, the entire vapor condensate is stripped once with inert gas when producing polyethylene terephthalate. By means of a three-stage reverse osmosis, a water/ethylene glycol mixture as concentrate and water as permeate are then produced from the prepurified vapor condensate. The concentrate is recirculated to the process of producing PET, and the permeate is used as demineralized water. The exhaust air of the stripper and the exhaust air of the condensers of the process columns are combined and by means of a blower supplied to the heat transfer furnace for thermal combustion.
It is disadvantageous that in the above-described processes the composition of the exhaust air of the stripper is subject to considerable variations as a result of changing concentrations of the organic components in the vapor condensate. A buffer function does not exist. An improvement of the recovery of ethylene glycol is only possible in the case of the process of producing PET, in which the stripped prepurified vapor condensate is subjected to a triple reverse osmosis. But since sticky, oily masses frequently occur in the vapor condensate, a blocking of the membranes of the reverse osmosis takes place after some time, as is assumed. Moreover, the combustion of the exhaust air of the stripper in the heat transfer furnace provides a direct coupling of the process of producing PET with the heat transfer furnace, so that malfunctions occurring in the process have a direct influence on the operation of the heat transfer furnace, and malfunctions in the heat transfer furnace have a direct influence on the production of PET.